1. Field of the Invention
The present invention relates generally to a trench device realized in a III-nitride material system, and relates more particularly to a class of trench switching devices that are nominally off in a III-nitride material system.
2. Description of Related Art
III-nitride semiconductors are presently known that exhibit a large dielectric breakdown field of greater than 2.2 MV/cm. III-nitride heterojunction structures are also capable of carrying extremely high currents, which makes devices fabricated in the III-nitride material system excellent for power applications.
Development of devices based on III-nitride materials has generally been aimed at high power-high frequency applications such as emitters for cell phone base stations. The devices fabricated for these types of applications are based on general device structures that exhibit high electron mobility and are referred to variously as heterojunction field effect transistors (HFETs), high electron mobility transistors (HEMTs) or modulation doped field effect transistors (MODFETs). These types of devices are typically able to withstand high voltages such as in the range of 100 Volts, while operating at high frequencies, typically in the range of 2-100 GHz. These types of devices may be modified for a number of types of applications, but typically operate through the use of piezoelectric polarization fields to generate a two dimensional electron gas (2DEG) that allows transport of very high current densities with very low resistive losses. The 2DEG is formed at an interface of AlGaN and GaN materials in these conventional III-nitride HEMT devices. Due to the nature of the AlGaN/GaN interface, and the formation of the 2DEG at the interface, devices that are formed in the III-nitride materials system tend to be nominally on, or depletion mode devices. The high electron mobility of the 2DEG at the interface of the AlGaN/GaN layers permits the III-nitride device, such as a HEMT device, to conduct without the application of a gate potential. The nominally on nature of the HEMT devices previously fabricated have limited their applicability to power management. The limitations of nominally on power devices is observed in the need to have a control circuit powered and operational, before power can be safely controlled by a nominally on device. Accordingly, it would be desirable to create a III-nitride heterojunction device that is nominally off to avoid current conduction problems during start-up and other modes.
A drawback of III-nitride devices that permit high current densities with low resistive losses is the limited thickness that can be achieved in the strained AlGaN/GaN system. The difference in the lattice structures of these types of materials produces a strain that can result in dislocation of films grown to produce the different layers. This results in high levels of leakage through a barrier layer, for example. Some previous designs have focused on reducing the in-plane lattice constant of the AlGaN layer to near where the point of relaxation occurs to reduce the dislocation generation and leakage. However, the problem of limited thickness is not addressed by these designs.
Another solution is to add insulation layers to prevent leakage problems. The addition of an insulator layer can reduce the leakage through the barrier, and typical layers used for this purpose are silicon oxide, silicon nitride, saphire, or other insulators, disposed between the AlGaN and metal gate layers. This type of device is often referred to as a MISHFET and has some advantages over the traditional devices that do not have an insulator layer.
While additional insulator layers can permit thicker strained AlGaN/GaN systems to be constructed, the confinement layer produced by the additional insulator results in lower current carrying capacity due to the scattering effect produced on electrons at the GaN/insulator interface. Also, the additional interface between the AlGaN layer and the insulator results in the production of interface trap states that slow the response of the device. The additional thickness of the oxide, plus the additional interfaces between the two layers, also results in the use of larger gate drive voltages to switch the device.
Conventional device designs using nitride material to obtain nominally off devices rely on this additional insulator to act as a confinement layer, and may reduce or eliminate the top AlGaN layer. These devices, however, typically have lower current carrying capacity due to scattering at the GaN/insulator interface.
Accordingly, it would be desirable to produce a heterojunction device or FET that has a low leakage characteristic with fewer interfaces and layers that can still withstand high voltage and produce high current densities with low resistive losses. Presently, planar devices have been fabricated with GaN and AlGaN alloys through a number of techniques, including MOCVD (metal organic chemical vapor deposition) as well as molecular beam epitaxy (MBE) and hydride vapor phase epitaxy (HVPE).
Materials in the gallium nitride material system may include gallium nitride (GaN) and its alloys such as aluminum gallium nitride (AlGaN), indium gallium nitride (InGaN) and indium aluminum gallium nitride (InAlGaN). These materials are semiconductor compounds that have a relatively wide direct bandgap that permits highly energetic electronic transitions to occur. Gallium nitride materials have been formed on a number of different substrates including silicon carbide (SiC), saphire and silicon. Silicon substrates are readily available and relatively inexpensive, and silicon processing technology has been well developed.
However, forming gallium nitride materials on silicon substrates to produce semiconductor devices presents challenges that arise from differences in the lattice constant, thermal expansion and bandgap between silicon and gallium nitride. The problems attendant with the lattice mismatch between GaN and traditional substrate materials are also prevalent in material layer structures involving GaN and GaN alloys. For example, GaN and AlGaN materials have lattice structures that differ significantly enough to produce interface strain between the layers, contributing to piezoelectric polarization. In many previous devices, the fields generated by the piezoelectric polarization are controlled to improve the characteristics of the devices. Variations in the content of aluminum in the AlGaN/GaN layer structures tends to vary the lattice mismatch between the materials to achieve different device characteristics, such as improved conductivity or isolation barriers.
A number of types of power devices can potentially benefit from a nominally off device with low on resistance. For example, it would be desirable to obtain a power switch, power rectifier, synchronous rectifier, current control device or other power devices that are nominally off when no power is applied. Current control devices can include diodes, pinch resistors, Schottky diodes and the like.
Trench structure semiconductor devices have been available in silicon for a number of years. Often, vertical conduction devices are realized with trench structures that can be formed in silicon as enhancement mode or nominally off devices. The devices, such as a MOSFET switch, typically operate by introducing an electric potential on a gate electrode to form an invertible channel along trench sidewalls, that provides a conductive path for the device. However, the conductive path typically has a given on resistance that is associated with the device voltage rating. For example, the thickness, composition and doping of the device materials contributes to determining device characteristics. These design parameters are manipulated to obtain desired characteristics, but on-resistance and blocking voltage for a given current rated device continue to receive attention for improvements to power semiconductor devices. It would be desirable to obtain a vertical conduction device with reduced on resistance that is capable of blocking large voltages.
One factor that contributes to the breakdown voltage value for a given device is the dielectric breakdown value for a given dielectric in the power semiconductor. For example, in silicon semiconductors, native oxides are available, such as silicon dioxide, that can serve as a suitable gate dielectric. However, no material equivalent to the native oxides for silicon is available for suitable gate dielectrics in the III-nitride material system. In addition, gate dielectric materials that would otherwise be suitable in silicon semiconductors, for example, do not transfer well to III-nitride devices. For example, if silicon dioxide or silicon nitride were to be used for a gate dielectric in a III-nitride device, these conventional dielectrics would rupture or otherwise fail. Typically, the large dielectric breakdown field produced in the III-nitride material system causes large electric fields in the III-nitride semiconductor devices that are greater than can be withstood with conventional dielectric materials.
It would be desirable to obtain a device structure and gate dielectric material suitable for use in a semiconductor device that experiences high electrical fields, without the dielectric material breaking down. It would also be desirable to obtain such a dielectric that is suitable for use in the III-nitride material system.